Many species of birds have feathers that are brilliantly colored without the use of pigments. In these cases, light of specific wavelengths is selectively scattered from nanostructures with variations in index of refraction on length-scales of the order of visible light. This phenomenon is called structural color. Structural color is distinct from pigmentary color in that it does not rely on the absorption of light.
Structural color arises from constructive interference of light scattered by variations in the refractive index within a material. A naturally occurring example is opal, whose iridescence is a consequence of Bragg diffraction from its ordered internal arrangement of silica. Similar structural colors can be produced in synthetic systems. For example, artificial opals can be made from self-assembled colloidal crystals in which the particle spacing is on the order of the wavelength of light. In all such materials, the colors vary with the viewing angle because the resonance condition changes as the incident light direction varies with respect to the crystal orientation. This variation of color with angle is well-understood and can be predicted from photonic band theory. Empirical and experimental observation has shown that the color of the preferentially scattered wavelength is on the same order as the average distance between the scattering nanostructures. For nanoparticle arrays, therefore, the particle size determines the array spacing and therefore the color of the observed light.
Less well-understood—and less exploited—are materials in which the structural color does not vary with angle. A recently discovered example from nature is the bright blue plumage of the plum-throated cotinga, whose feathers are patterned with a dense, disordered arrangement of pores. The short-range correlations in the pore network give rise to constructive interference of scattered light. Because the structure is isotropic, the interference condition does not vary with orientation, and therefore the color is independent of the viewing angle. Synthetic materials with similar appearance can be made through a variety of approaches. Amorphous colloidal structures can be made by drying bidisperse mixtures of particles. Thin films of these disordered structures show angle-independent structural color. Similar systems, termed “photonic liquids” or “photonic glasses” can be made from suspensions of highly-charged spheres that, though monodisperse, can nonetheless form amorphous structures due to the soft, long-range electrostatic repulsion between the particles. An alternative approach that does not involve making a disordered system is to dope ordered structures (colloidal crystals) with nanoparticles, which act as scattering sites. The structural color in all of these materials—bird feathers, binary packings, photonic glasses, and nanoparticle-doped crystals—is not due to Bragg reflection, which requires a weakly scattering system with long-range order, but is instead a result of scattering with a strong wavelength dependence that arises from correlations in the colloidal structure.